This invention relates generally to color cathode ray tubes (CRTs) and is particularly directed to the measurement of electron beam convergence in a multi-beam CRT.
CRTs such as those used in television receivers and computer terminals are generally provided with a plurality of luminescent elements deposited upon the inner surface of the CRT's faceplate upon which a video image is displayed. Impingement of energetic electrons upon the luminescent elements, which are commonly termed "phosphor dots", results in light output from these luminescent elements. Near simultaneous illumination of large numbers of phosphor dots in a predetermined array results in the display upon the CRT's faceplate of a desired alphanumeric character or graphic image. Positioned immediately adjacent to the faceplate and within the CRT is a structure generally termed a "shadow mask" having a large number of apertures therein through which the energetic electrons transit as they are directed toward impact with the phosphor dots. In a color CRT, wherein three electron guns are positioned in close proximity to one another in the neck or rear portion of the CRT, each aperture of the shadow mask corresponds with a trio of phosphor dots on the faceplate which respectively emit red, green and blue light, the primary colors, when struck by energetic electrons. Each aperture in the shadow mask is aligned with the three electron guns and the three associated, grouped phosphor dots to permit only electrons from the red electron gun to be incident upon red phosphor dots. Similarly, electrons from the green electron gun illuminate green phosphor dots, and the blue electron gun illuminates blue phosphor dots. In order to avoid deleterious moire effects, a scanning electron beam from any one of the three primary color guns must simultaneously illuminate several dots of the appropriate color. If the green gun (usually the center gun in an in-line configuration) illuminates red or blue dots a color contamination or loss of "purity" is said to result. Similar purity errors may occur with the other two primary colors. When the yoke and purity magnets on the neck are adjusted for proper purity, it often happens that another defect called misconvergence remains in portions of the display field. Misconvergence is a lateral displacement on the screen of the center of one of the three primary color beams with respect to the center of one of the other primary color beams. If, for example, the red and blue beams were misconverged, a desired magenta character "o" would appear as offset red and blue ovals. Using permanent magnets on the CRT neck, it is easy to obtain excellent convergence in the center of the display field. It is difficult, however, to obtain satisfactory convergence in all regions of a high resolution display. The objective measurement of convergence errors is the subject of the present invention.
Prior art methods of convergence measurement have involved cumbersome counting and identification of illuminated microscopic dots, or have required somewhat subjective adjustment of motion devices such as prisms for superimposing the misconverged primary color images. Electronic methods of convergence measurement can be found in U.S. Pat. No. 4,441,120 to Gerritsen. In this patent there is disclosed use of a quadrant-type photodetector for measuring the position of a line image. The Gerritsen method requires, however, one of the following somewhat objectionable techniques: precise mechanical adjustment of the split detector relative to a line image, or adjustment of synchronization timing signals to move the line image, or other deflection means to move the line image. The present invention avoids all of these troublesome techniques, using instead computer-controlled interaction between a split photodetector and the video signals of a bit-mapped or appropriate character-mapped display.
The present invention involves neither the physical displacement of the photosensitive detector nor the precise positioning upon the CRT's faceplate of either the photosensitive detector or the illuminated lines from which the convergence measurements are made. In accordance with the present invention, CRT color convergence measurements are made by means of a stationary, computer controlled quad detector capable of accurately detecting individual horizontal and vertical lines and of precisely measuring differences in position among the three lines representing the primary colors of red, green and blue. This measurement procedure eliminates inaccuracies arising from the staggered array of the phosphor dots on the CRT's faceplate as well as human errors involved in the positioning of the photosensitive detector and observing the relative positions of the various color lines.